Method for sealing a fuel cell

ABSTRACT

The invention relates to a method for sealing a fuel cell ( 5 ) and to a fuel cell ( 5 ) which is produced using such a method. The fuel cell ( 5 ) has at least one membrane-electrode unit ( 42 ) and bipolar plates ( 18, 22 ). The method has the steps of attaching a seal material ( 54 ) to at least one side of the membrane-electrode unit ( 42 ) in a bonded manner, attaching a precursor ( 62 ) to seal points ( 58 ) of the at least one bipolar plate ( 18, 22 ), placing the at least one bipolar plate ( 18, 22 ) on the membrane-electrode unit ( 42 ) such that the seal points ( 58 ) together with the precursor ( 62 ) come into contact with the seal material ( 54 ), and pressing the at least one bipolar plate ( 18, 22 ) and the membrane-electrode unit ( 42 ) together under the effect of pressure and/or temperature such that the seal material ( 54 ) forms a bonded connection to the at least one bipolar plate ( 18, 22 ) and to the membrane electrode unit ( 42 ).

BACKGROUND OF THE INVENTION

The present invention relates to a method for sealing a fuel cell, and afuel cell which is sealed by means of such a method.

In fuel cell systems, the oxidizing agent—oxygen from the ambient air—isgenerally used to react with hydrogen in the fuel cell to produce waterand therefore to provide electrical power through electrochemicalconversion. Seals are arranged in the fuel cell to isolate an inner partof the fuel cell from the environment.

A method for sealing a coolant chamber of a bipolar plate of a fuel cellis known from DE 10 2012 221 730 A1. In this method, a seal for sealingthe gas chamber is positioned such that both bipolar plate halves of thebipolar plate are contacted by the seal.

The background of the invention consists in that, in the case of a fuelcell stack, the greatest risk of a defect during the manufacture of thestack comes from the sealing points in the cell. The overall risk ofdefects with a total of several 1000 sealing points per stack increasesconsiderably so that, after completion of the fuel cell stack, arelatively high reject rate of defective fuel cell stacks in the regionof 10% arises. The high reject rate of defective fuel cell stackstherefore increases the price of the defect-free fuel cell stack. A lotof effort is therefore spent on ensuring the leak-tightness of theseals. This increases the price of the fuel cell stack considerably.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to specify a method forsealing a fuel cell, with which the sealing properties of the sealingpoints are improved and which enables simpler and therefore moreeconomical production of the seals.

In this case, the inventive method for sealing a fuel cell comprises thesteps of applying a sealing material to at least one side of themembrane-electrode unit in a bonded manner, applying a precursor tosealing points of the at least one bipolar plate, placing the at leastone bipolar plate on the membrane-electrode unit so that the sealingpoints with the precursor come into contact with the sealing material,and pressing the at least one bipolar plate together with themembrane-electrode unit under pressure and/or under temperature so thatthe sealing material forms a bonded connection to the at least onebipolar plate and the membrane electrode unit.

A precursor in the sense of the invention is something which happens orexists before something else and has an influence thereon. In this case,the precursor essentially determines the future sealing properties ofthe sealing material.

The inventive method for sealing a fuel cell specifies a simple methodwith which the sealing material can be connected to themembrane-electrode unit and the bipolar plates in a bonded manner.Economical manufacture of such a seal is thus possible.

In addition, a seal produced in such a way is advantageous in that ahigh degree of leak-tightness is enabled. The reject rate of the fuelcells due to inadequate sealing can therefore be substantially reduced.In addition, the costs for producing such a fuel cell are thereforereduced.

In a preferred embodiment of the invention, a primer or a vulcanizingagent is used as the precursor. Primers are bonding agents with whichthe adhesive properties of surfaces are improved. The sealing propertiesof a sealing material can thus be improved.

A vulcanizing agent is an agent with which a chemical process forconverting rubber or related polymers into more durable materials takesplace, for example, via the addition of sulfur. These additions changethe polymer through the formation of cross-linkages (bridges) betweenthe individual polymer chains. A particularly durable connection to thebipolar plates is thus formed, whereby the sealing properties aresubstantially improved.

In a further preferred embodiment of the invention, FKM or EPDM is usedas the sealing material. EPDM has a very good aging resistance.Moreover, EPDM is resistant to oxygen. FKM has a good heat resistance, alow gas permeability and an excellent aging resistance. A sealingmaterial with excellent sealing properties can therefore be producedusing FKM or EPDM.

In an advantageous configuration of the invention, a bead is formed onthe bipolar plate before or after the application of the precursor. Abead in the sense of the invention is a manually or mechanicallyproduced channel-like depression. This bead is advantageous in that therigidity of the bipolar plate is increased so that a sufficiently highpressure force can be applied to the sealing material after the bracingof the bipolar plates.

Moreover, a cross-section of a region to be sealed is reduced so thatthe bead forms a metal barrier over a significant part of the sealingheight. The gas diffusion through the sealing material is thus reduced.In addition, the bead has the same coefficient of thermal expansion asthe base material of the bipolar plate. The thermal properties are thusimproved so that thermal expansion of the bead results in an additionalpressing-together at the sealing points and thereby increases theleak-tightness of the sealing points.

The sealing material is preferably applied by printing. The applicationof the sealing material by printing can be easily incorporated as partof an automated manufacturing process so that a sealing material appliedin this way can be economically produced.

The sealing material is particularly preferably applied by means of ascreen printing technique. A high-quality sealing material can beefficiently batch-produced using a screen printing technique. Thesealing properties of a sealing material produced thereby are thereforeimproved. Moreover, with a high number of units, a sealing materialproduced in this way can be produced economically.

In an advantageous further development, the sealing material is appliedby spraying. The proposed application of sealing material by sprayingcontributes significantly to the cost-effective manufacture.

The object of the invention is additionally achieved by a fuel cell,which is sealed according to the inventive method. In this case, thefuel cell comprises at least one membrane-electrode unit, to which asealing material is applied in a bonded manner, and bipolar plates,which have sealing points which abut against the sealing material of themembrane-electrode unit and form a sealing connection, wherein aprecursor is applied to the sealing points of at least one bipolar plateso that a bonded connection is formed between the sealing material andthe at least one bipolar plate.

The advantages mentioned with respect to the method can be achieved bysuch a fuel cell, which is sealed according to the inventive method.

In a preferred exemplary embodiment, the precursor is a primer or avulcanizing agent. In a further preferred exemplary embodiment, thesealing material is FKM or EPDM. The advantages already mentioned withrespect to the method can therefore be achieved.

The bipolar plates preferably have a bead at the sealing points. In thiscase, the beads have the advantages already mentioned with respect tothe method.

The object of the invention is moreover achieved by a fuel cell stackhaving at least one fuel cell, which is sealed according to theinventive method. The invention additionally specifies a motor vehiclehaving such a fuel cell stack. The above-mentioned advantages can beachieved with such a fuel cell stack or a motor vehicle which has such afuel cell stack.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is illustrated in the drawingand explained in more detail in the description below.

FIG. 1 shows a sectional view of an exemplary embodiment of a fuel cellwhich is sealed according to the inventive method.

DETAILED DESCRIPTION

FIG. 1 shows a sectional view of an exemplary embodiment of a fuel cell5 which is sealed according to the inventive method. This method isexplained with reference to a detail of an individual cell of the fuelcell 5. The fuel cell 5 is formed from a cathode bipolar plate 18 havinga bead 14. An anode bipolar plate 22 is arranged in a mirror-invertedmanner with respect to the cathode bipolar plate 18 so that the beads 14of both bipolar plates 18, 22 are opposite one another. Channels 26 arethus formed between the cathode bipolar plate 18 and the anode bipolarplate 22.

A layered structure 30 is arranged within the channels 26. This layeredstructure 30 is shown by way of example in a channel 26. In this case,the layered structure 30 is composed of a first and a second gasdiffusion layer 34, 38, which are separated by a membrane-electrode unit42. The oxygen 46 which is required for the oxidation flows between thefirst gas diffusion layer 34 and the cathode bipolar plate 18. Thehydrogen 50 which is preferably used as fuel flows between the secondgas diffusion layer 38 and the anode bipolar plate 22.

A sealing material 54 is applied to the membrane-electrode unit 42 in abonded manner at the points at which the beads 14 of the cathode bipolarplate 18 and the anode bipolar plate 22 are opposite one another.Sealing points 58 are formed in regions of the beads 14 which come intocontact with the sealing material 54. A precursor 62 has been applied tothese sealing points 58 before the placement of the bipolar plates 18,22.

As already described above, most defects arise at the sealing points 58.Leaks in such a fuel cell 5 occur at these sealing points 58 as a resultof inadequate contact.

In a next step, the bipolar plates 18, 22 are pressed together under ahigh temperature and pressure, so that a bonded connection forms betweenthe sealing material 54 and the respective bipolar plate 18, 22 at thesealing points 58 where the precursor 62 is applied.

1. A method for sealing a fuel cell (5), wherein the fuel cell (5) has amembrane-electrode unit (42) and a bipolar plate (18, 22), the methodcomprising the following steps: applying a sealing material (54) to atleast one side of the membrane-electrode unit (42) in a bonded manner,applying a precursor (62) to sealing points (58) of the bipolar plate(18, 22), placing the bipolar plate (18, 22) on the membrane-electrodeunit (42) so that the sealing points (58) with the precursor (62) comeinto contact with the sealing material (54), and pressing the bipolarplate (18, 22) together with the membrane-electrode unit (42) underpressure and/or under temperature so that the sealing material (54)forms a bonded connection to the bipolar plate (18, 22) and the membraneelectrode unit (42).
 2. The method for sealing a fuel cell (5) asclaimed in claim 1, characterized in that a primer or a vulcanizingagent is used as the precursor (62).
 3. The method for sealing a fuelcell (5) as claimed in claim 1, characterized in that FKM or EPDM isused as the sealing material (54).
 4. The method for sealing a fuel cell(5) as claimed in claim 1, characterized in that a bead (14) is formedon the bipolar plate (18, 22) before or after the application of theprecursor (62).
 5. The method for sealing a fuel cell (5) as claimed inclaim 1, characterized in that the sealing material (54) is applied byprinting.
 6. The method for sealing a fuel cell (5) as claimed in claim5, characterized in that the sealing material (54) is applied by ascreen printing technique.
 7. The method for sealing a fuel cell (5) asclaimed in claim 1, characterized in that the sealing material (54) isapplied by spraying.
 8. A fuel cell (5), which is sealed by means of amethod for sealing as claimed in claim 1, wherein the fuel cell (5)comprises: at least one membrane-electrode unit (42), to which a sealingmaterial (54) is applied in a bonded manner, and bipolar plates (18,22), which have sealing points (58) which abut against the sealingmaterial (54) of the membrane-electrode unit (42) and form a sealingconnection, wherein a precursor (62) is applied to the sealing points(58) of at least one bipolar plate (18, 22) so that a bonded connectionis formed between the sealing material (54) and the at least one bipolarplate (18, 22).
 9. The fuel cell (5) as claimed in claim 8,characterized in that the precursor (62) is a primer or a vulcanizingagent.
 10. The fuel cell (5) as claimed in claim 8, characterized inthat the sealing material (54) is FKM or EPDM.
 11. The fuel cell (5) asclaimed in claim 8, characterized in that the bipolar plates (18, 22)have a bead (14) at the sealing points (58).
 12. A fuel cell stackhaving at least one fuel cell (5) as claimed in claim
 8. 13. A motorvehicle having a fuel cell stack as claimed in claim
 12. 14. The methodfor sealing a fuel cell (5) as claimed in claim 2, characterized in thatFKM or EPDM is used as the sealing material (54).
 15. The method forsealing a fuel cell (5) as claimed in claim 14, characterized in that abead (14) is formed on the bipolar plate (18, 22) before or after theapplication of the precursor (62).
 16. The method for sealing a fuelcell (5) as claimed in claim 15, characterized in that the sealingmaterial (54) is applied by printing.
 17. The method for sealing a fuelcell (5) as claimed in claim 16, characterized in that the sealingmaterial (54) is applied by a screen printing technique.
 18. The methodfor sealing a fuel cell (5) as claimed in claim 17, characterized inthat the sealing material (54) is applied by spraying.